1. Field of the Invention
The present invention generally relates to projecting two-dimensional images with reduced speckle noise on display screens.
2. Description of the Related Art
It is generally known to project a two-dimensional image on a display screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a laser beam over a raster pattern. However, the known image projection systems project an image of limited resolution, typically less than a fourth of video-graphics-array (VGA) quality of 640×480 pixels, and with speckle noise. As such, the known projection systems and display screens have limited versatility.
Speckle noise is an inherent problem in laser-based projection systems and causes considerable degradation in image quality. A monochromatic (red, blue or green) laser emits a laser beam having coherent waves of the same frequency and also having spatial coherence, that is, the waves have a fixed phase relationship with one another both in space and in time. When the beam is incident on a diffuse screen, the waves are scattered by being reflected from the screen and/or transmitted through the screen. The scattered waves have random phase delays and propagate along different directions, but all have the same frequency. When such scattered waves meet, for example, at the retina of the human eye, they produce a static distribution of constructive and destructive interference, i.e., an interference pattern, also known as speckle noise. The human eye whose integration time is on the order of tens of milliseconds sees the speckle noise as a degraded image. If the laser beam does not have entirely coherent waves, then their phase delays can change substantially during the time that the scattered waves take to negotiate the screen and, as a result, the speckle noise pattern changes as well during the integration time of the human eye, thereby reducing speckle contrast.
Each coherent wave of the laser beam is coherent with itself, a property known as temporal coherence. If a wave is combined with a delayed copy of itself, as in a Michelson interferometer, the duration of the delay over which is produces interference is known as the coherence time of the wave. A corresponding property known as coherence length is calculated by multiplying the coherence time by the speed of light. The coherence time of the laser is inversely proportional to its bandwidth. A truly monochromatic wave has an infinite coherence time and an infinite coherence length. However, in practice, no wave is truly monochromatic since this requires a wave of infinite duration. By way of example, a stabilized helium-neon laser can produce a laser beam with a coherence length in excess of five meters, and the coherence time is on the order of several nanoseconds. Hence, the coherence time of lasers is generally in the nanosecond range, while the laser beam takes only a few picoseconds to reflect from and/or pass through the display screen.